Electrical submersible pump assemblies (ESP) for oil wells commonly include an electrical motor, a seal section, and a centrifugal pump. The seal section equalizes the pressure of lubricant within the motor with the well fluid hydrostatic pressure. The motor rotates a shaft that is part of a shaft assembly extending through the seal section and the pump. A rotary gas separator may also be located in the assembly.
The shafts that make up the shaft assemblies can be lengthy, 30 feet or more. Radial bearings in the motor, seal section and pump provide radial support for the shafts of the shaft assembly. The bearings come in sets. One part, often called the bushing, is pressed into a stationary, non-rotating part of the ESP. The other part, often called a sleeve, is fitted onto the shaft for rotation in unison with the shaft. The sleeve and shaft have corresponding keyways fitted with a common key between each other. The keys and keyways transmit the rotation of the shaft to the sleeve.
The ESP has other components that are mounted to the shaft for rotation, such as impellers within the pump. Each impeller has a hub or sleeve that has a matting keyway with the shaft for rotation therewith. Protective sleeves and spacers may also be mounted around the shaft of the pump for rotation with the shaft.
ESP shafts are formed of steel alloys, such as carbon steel, Inconel and Monel. The sleeves are often formed of similar materials. Alternately, ESPs may use tungsten carbide or ceramic bearing, sleeves, impeller hubs and pump stage thrust bearings for certain applications. The purpose is to reduce wear, particularly if abrasives are in the fluids that immerse these components, which will be referred to herein as abrasive resistant (AR) components. The material of AR components is harder than the shafts of the motor, seal section or pump.
One problem that may occur with AR components results from the mating keyway formed in the AR component. The keyway will produce a stress concentration factor that can cause the AR component to crack. Another problem with AR components can arise from thermal expansion. The steel alloy shafts have a much higher coefficient of thermal expansion than either carbide or ceramic materials used in AR components. Because of the differences in thermal expansion, excessive clearances need to be provided between the shaft and AR component. The clearance allows the shaft and AR component to thermally expand during operating conditions. Once the ESP is at full operating temperature, the clearance reduces due to the different thermal expansion coefficients. The excessive clearance that exists between the shaft and the AR component before the shaft and sleeve reach the full operating temperature can result in looseness at startup that may cause excessive vibration until reaching the full operating temperature. The higher the operating temperature, the greater the initial clearance must be. If the initial clearance is sufficiently large, mechanical damage can occur during startup before the system has time to expand.